The disclosure relates generally to floating offshore structures. More particularly, the disclosure relates to buoyant semi-submersible offshore platforms for offshore drilling and production operations. Still more particular, the disclosure relates to the geometry of the hull, columns, and pontoons of semi-submersible offshore platforms.
Most conventional semi-submersible offshore platforms include a hull with sufficient buoyancy to support a work deck above the water. For example, FIGS. 1 and 2 illustrate a conventional semi-submersible platform 10 deployed in a body of water 11. Platform 10 includes a buoyant hull 20 and a topsides or deck 30 supported by hull 20 above the surface 12 of water 11. The hull 20 typically includes a plurality of vertical upstanding columns 21 and a plurality of horizontal pontoons 22 extending between columns 21. The deck 30 sits atop the upper ends of columns 21. In general, the size of the pontoons 22 and the number of columns 21 are governed by the size and weight of the deck 30 and equipment disposed on deck 30. As with most conventional semi-submersible platforms, each column 21 of platform 10 has a constant or uniform width W21 in side view moving vertically between deck 30 and pontoons 22, and each pontoon 22 of platform 10 has a constant or uniform width W22 in top view moving horizontally between adjacent columns 21.
The hull 20 is typically divided into several closed compartments, each compartment having a buoyancy that can be adjusted for purposes of flotation and trim. Typically, a pumping system pumps ballast water into and out of the compartments to adjust their buoyancy. The compartments are typically defined by horizontal and/or vertical bulkheads in the pontoons 22 and columns 21. Normally, the compartments of the pontoon 22 and the lower compartments of the columns 21 are filled with water ballast when the platform is in its operational configuration, and the upper compartments of the columns 21 provide buoyancy for the platform 10.
Typically, piping or risers are hung from the platform, and thus, the hull must be sufficiently buoyant to support the deck as well as any piping or risers. The relatively large heave (vertical) motions experienced by many conventional semi-submersible platforms usually dictate the use of steel “catenary” risers (SCRs) that extend between the platform and the seafloor, and the positioning of wellhead equipment such as the production tree at the sea-floor (i.e., a “wet” tree), rather than on the platform. The catenary shape of SCRs accommodate and absorb the large heave motions and horizontal motions of the floating semi-submersible platform.
The “draft” of a floating offshore structure is defined as the vertical distance measured from the waterline (i.e., the surface of the water) to the bottom of the hull. For example, in FIG. 2, semi-submersible platform 10 has a draft D10 measured from the bottom of hull 20 to the surface 12. A semi-submersible offshore platform having a draft less than 100 ft. is typically described as “shallow” draft. Increasing the draft of a semi-submersible offshore platform can reduce heave motions (i.e., movement in the vertical direction) as the pontoons at a greater depth below the surface of the water where wave excitation forces are generally lower. Accordingly, semi-submersible platforms having a draft greater than 100 ft., often described as “deep” draft, usually experience smaller heave motions as compared to shallow draft semi-submersible platforms.
The draft of a semi-submersible platform is increased by lengthen the columns of the hull. Although this may reduce heave motions by positioning the pontoons at greater depths, longer columns are more susceptible to a phenomenon known in the art as “vortex-induced-motion” (VIM). In particular, a boundary layer forms close to the outer surface of a body exposed to a moving fluid due to viscous forces. Separation in the flow of the moving fluid occurs when the boundary layer reaches certain points behind a blunt body such as a column on a semi-submersible platform. The fluid flow becomes detached from the surface of the object and takes the form of eddies and vortices. Oscillating flow characterized by periodic vortex shedding may take place when the fluid flows past the body at certain velocities, depending on the size and shape of the body. The undesirable resonance motion of a moored floating platform caused by vortex shedding effects is called VIM “lock-in.” On deep draft semi-submersible platforms with longer columns, VIM excitation forces are typically higher than those on conventional semi-submersibles with shorter columns, and hence, deep draft semi-submersible platforms are more likely to experience larger VIM motions and VIM lock-in. VIM is a significant contributor to fatigue damage of offshore structures such as platforms, mooring lines, and risers. In addition, VIM induced motions may render it more difficult to maintain the lateral position of the offshore platform over the well site and/or increase the likelihood of damaging riser systems.
The location of final assembly of a semi-submersible offshore platform may involve integration of the hull and topsides at the shipyard (i.e., quayside), at a nearshore location, or at the operation site (i.e., the location where drilling and/or production will occur). For quayside integration, the topsides is lifted and mounted to the hull with heavy lifting equipment (e.g., heavy lift crane) in the shipyard. For nearshore integration, the topsides is lifted and mounted to the hull with heavy lift cranes or heavy lift barge in the water close to the shore. For integration at the operation site, the hull is transported offshore to the operation site, either by towing it at a shallow draft, or by floating it aboard a heavy lift vessel. At the operation site, the hull is ballasted, and the topsides is then either lifted onto the tops of the columns by heavy lift cranes carried aboard a heavy lift barge, or by floating the work platform over the top of the partially submerged hull using a deck barge. In either case, the procedure is typically effected far offshore (e.g., 100 miles, or 161 km), is performed in open seas, and is strongly dependent on weather conditions and the availability of a heavy lift barge, making it both risky and expensive.
Quayside topsides integration in the shipyard is usually the safest and most economical among the three integration options. However, quayside water depths are usually on the order of about 30-35 ft., and thus, for quayside integration, the hull must provide sufficient buoyancy to support its own weight and topside weight while maintaining a draft less than 30-35 ft. It may be challenging to maintain such a shallow draft at the quayside location with semi-submersible platforms designed for deep draft deployment at the operation site—due to the lack of sufficient buoyancy provided by conventional semi-submersible platform geometries at this shallow draft.
After hull and topsides integrations quayside or near shore, the semi-submersible platform is transported to the operation site by wet tow or with a heavy transportation vessel. Both methods involve ballasting down the hull during pre-service operations. During the ballasting process, the stability of the floating structure typically decreases as the draft increases and the pontoons transition from being partially submerged to wholly submerged. This may be particularly problematic with deep draft semi-submersibles due to the length of the columns and the height of the topsides supported by the columns.